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Tue, 12 May 2026 07:35:42 -0700 (PDT) Date: Tue, 12 May 2026 14:35:42 +0000 From: Wei Yang To: "David Hildenbrand (Arm)" Cc: Balbir Singh , Wei Yang , akpm@linux-foundation.org, ljs@kernel.org, riel@surriel.com, liam@infradead.org, vbabka@kernel.org, harry@kernel.org, jannh@google.com, sj@kernel.org, ziy@nvidia.com, linux-mm@kvack.org, Lorenzo Stoakes , stable@vger.kernel.org Subject: Re: [PATCH] mm/page_vma_mapped: revalidate and do proper check before return device-private pmd Message-ID: <20260512143542.izpp3gu4iqxttw3f@master> Reply-To: Wei Yang References: <20260508013728.21285-1-richard.weiyang@gmail.com> <5e9ee072-b927-41e0-ba98-c9fdf11eccbc@nvidia.com> <0aab59b8-71c5-4059-8281-5dd876946528@kernel.org> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <0aab59b8-71c5-4059-8281-5dd876946528@kernel.org> User-Agent: NeoMutt/20170113 (1.7.2) On Tue, May 12, 2026 at 02:43:54PM +0200, David Hildenbrand (Arm) wrote: >On 5/9/26 00:48, Balbir Singh wrote: >> On 5/8/26 11:37, Wei Yang wrote: >>> For pmd_trans_huge() and pmd_is_migration_entry(), we does following >>> before return the pmd entry: >>> >>> * re-validate pmd entry >>> * check PVMW_MIGRATION >>> * check_pmd() >>> * handle on pte level if split under us >>> >>> But for device-private pmd, we just return after pmd_lock(). This may >>> lead to inproper situation. >>> >> >> Could you elaborate a more on the improper situation? >> >>> This patch fixes commit 65edfda6f3f2 ("mm/rmap: extend rmap and migration >>> support device-private entries") by following the same pattern as >>> pmd_trans_huge() and pmd_is_migration_entry(). >>> >>> Fixes: 65edfda6f3f2 ("mm/rmap: extend rmap and migration support device-private entries") >>> Signed-off-by: Wei Yang >>> Cc: David Hildenbrand >>> Cc: Balbir Singh >>> Cc: SeongJae Park >>> Cc: Zi Yan >>> Cc: Lorenzo Stoakes >>> Cc: >>> --- >>> mm/page_vma_mapped.c | 34 +++++++++++++++++++++++----------- >>> 1 file changed, 23 insertions(+), 11 deletions(-) >>> >>> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c >>> index a4d52fdb3056..5d337ea43019 100644 >>> --- a/mm/page_vma_mapped.c >>> +++ b/mm/page_vma_mapped.c >>> @@ -269,21 +269,33 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) >>> spin_unlock(pvmw->ptl); >>> pvmw->ptl = NULL; >>> } else if (!pmd_present(pmde)) { >>> - const softleaf_t entry = softleaf_from_pmd(pmde); >>> + softleaf_t entry = softleaf_from_pmd(pmde); >>> >>> if (softleaf_is_device_private(entry)) { >>> pvmw->ptl = pmd_lock(mm, pvmw->pmd); >>> - return true; >>> - } >>> - >>> - if ((pvmw->flags & PVMW_SYNC) && >>> - thp_vma_suitable_order(vma, pvmw->address, >>> - PMD_ORDER) && >>> - (pvmw->nr_pages >= HPAGE_PMD_NR)) >>> - sync_with_folio_pmd_zap(mm, pvmw->pmd); >>> + entry = softleaf_from_pmd(*pvmw->pmd); >>> + >>> + if (softleaf_is_device_private(entry)) { >> >> Do we need to check softleaf_is_device_private() twice, can't we hold the pmd >> lock and check once? > >I think what we try to do here is, is to only grab the lock if we verified that there is something of interest in there. > >I wonder if we should rewrite that whole thing to just do a pmd_same() check after grabbing the lock. > >Something a lot cleaner like: > >diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c >index a4d52fdb3056..de6a255cc847 100644 >--- a/mm/page_vma_mapped.c >+++ b/mm/page_vma_mapped.c >@@ -242,40 +242,28 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) > */ > pmde = pmdp_get_lockless(pvmw->pmd); > >- if (pmd_trans_huge(pmde) || pmd_is_migration_entry(pmde)) { >- pvmw->ptl = pmd_lock(mm, pvmw->pmd); >- pmde = *pvmw->pmd; >- if (!pmd_present(pmde)) { >- softleaf_t entry; >- >- if (!thp_migration_supported() || >- !(pvmw->flags & PVMW_MIGRATION)) >- return not_found(pvmw); >- entry = softleaf_from_pmd(pmde); >- >- if (!softleaf_is_migration(entry) || >- !check_pmd(softleaf_to_pfn(entry), pvmw)) >- return not_found(pvmw); >- return true; >- } >- if (likely(pmd_trans_huge(pmde))) { >- if (pvmw->flags & PVMW_MIGRATION) >- return not_found(pvmw); >- if (!check_pmd(pmd_pfn(pmde), pvmw)) >- return not_found(pvmw); >- return true; >- } >- /* THP pmd was split under us: handle on pte level */ >- spin_unlock(pvmw->ptl); >- pvmw->ptl = NULL; >- } else if (!pmd_present(pmde)) { >- const softleaf_t entry = softleaf_from_pmd(pmde); >- >- if (softleaf_is_device_private(entry)) { >- pvmw->ptl = pmd_lock(mm, pvmw->pmd); >- return true; >- } >+ if (pmd_present(pmde)) { >+ if (!pmd_leaf(pmde)) >+ goto pte_table; >+ if (pvmw->flags & PVMW_MIGRATION) >+ return not_found(pvmw); >+ if (!check_pmd(pmd_pfn(pmde), pvmw)) >+ return not_found(pvmw); >+ } else if (pmd_is_migration_entry(pmde)) { >+ softleaf_t entry = softleaf_from_pmd(pmde); >+ >+ if (!(pvmw->flags & PVMW_MIGRATION)) >+ return not_found(pvmw); >+ if (!check_pmd(softleaf_to_pfn(entry), pvmw)) >+ return not_found(pvmw); >+ } else if (pmd_is_device_private_entry(pmde)) { >+ softleaf_t entry = softleaf_from_pmd(pmde); > >+ if (pvmw->flags & PVMW_MIGRATION) >+ return not_found(pvmw); >+ if (!check_pmd(softleaf_to_pfn(entry), pvmw)) >+ return not_found(pvmw); >+ } else { > if ((pvmw->flags & PVMW_SYNC) && > thp_vma_suitable_order(vma, pvmw->address, > PMD_ORDER) && >@@ -285,6 +273,15 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) > step_forward(pvmw, PMD_SIZE); > continue; > } >+ >+ /* Double-check under PTL that the PMD didn't change. */ >+ pvmw->ptl = pmd_lock(mm, pvmw->pmd); >+ if (pmd_same(pmde, pmdp_get(pvmw->pmd))) >+ return true; >+ spin_unlock(pvmw->ptl); >+ pvmw->ptl = NULL; >+ goto restart; >+pte_table: > if (!map_pte(pvmw, &pmde, &ptl)) { > if (!pvmw->pte) > > > > >There is likely room to clean this up / compress it further. I tried to compress above logic like this, hope it could look cleaner. if (pmd_trans_huge(pmde) || pmd_is_valid_softleaf(pmde)) { unsigned long pfn; bool is_migration = pmd_is_migration_entry(pmde); bool for_migration = !!(pvmw->flags & PVMW_MIGRATION); if (is_migration != for_migration) return not_found(pvmw); if (pmd_trans_huge(pmde)) pfn = pmd_pfn(pmde); else pfn = softleaf_to_pfn(softleaf_from_pmd(pmde)); if (!check_pmd(pfn, pvmw)) return not_found(pvmw); } else if (!pmd_present(pmde)) { >I'll note that this now also adds proper check_pmd() checks to pmd_is_device_private_entry(). > >The not_found(pvmw) if check_pmd() fails is rather weird ... but likely this works because >THPs can really only be mapped through one PMD, and we always will look at the right spot ... > >-- >Cheers, > >David -- Wei Yang Help you, Help me